The present invention relates, in general, to semiconductor devices and, more particularly, to a field effect transistor for measuring a change in surface potential as a function of temperature of the transistor's gate electrode due to exposure to a chemical as used in, for example, a chemical sensor.
Field effect transistors have been previously used in some cases as chemical sensors for measuring the concentration of a chemical in a fluid. One such prior sensor uses a gate electrode that is suspended over the channel region so as to provide a gap in which fluid may enter and contact an exposed surface of the gate electrode. A chemical in the fluid, to which the gate electrode is particularly sensitive, is adsorbed onto the exposed surface and changes the surface potential of the gate electrode. The drain current of the transistor changes in response to this surface potential change. Thus, if a constant gate voltage source is applied to the gate electrode during sensing, the change in drain current can be correlated to the concentration of the chemical in the fluid.
It has been found that the surface chemical reactions, which include adsorption/desorption reactions of the chemical to be sensed onto and off of the exposed gate electrode surface, of this prior sensor are very sensitive to temperature, so it is desirable that the temperature of the gate electrode be more directly regulated to optimize the output of the sensor. Also, it has been found to be desirable that this temperature be elevated above the ambient temperature to provide improved performance for the sensor. However, prior chemical sensors do not provide an integrated heating element for direct temperature control of the gate electrode. Instead, an external heater is required to heat the entire sensor assembly, rather than the gate electrode directly. Such an external heater is inconvenient to provide in a final, fully-manufactured chemical sensor assembly and increases the manufacturing cost thereof. Also, an external heater requires significant power consumption during operation.
Accordingly, there is a need for a chemical probe field effect transistor that includes a convenient and inexpensive heater that is directly incorporated into the sensor itself for direct, local heating of the sensor's gate electrode.